The prismatic structure of the bivalve hinge ligament has been determined for Spisula solidissima. The prisms are of two types: both contain aragonite (50 to 60 percent dry weight) as the sole mineral form. The remaining material is protein, which is elastic when wet. One type of prism is cylindrical, having crystals oriented parallel to the prism axis, and in shape and orientation are similar to dental enamel prisms. The other type of prism is cylindrical but constricted or noded at regular intervals, and the crystal spacing varies regularly along the prism axis. Thus, the noded prisms probably respond to compression like an elliptical spring and account to the elastic properties of the ligament. Low angle X-ray diffraction and electron microscopy data indicate the noded prisms are composed of lamellar concentric sheets of protein with the crystals imbedded longitudinally between the lamellae. The individual crystals (1000A diameter) are surrounded by a protein envelope (100A thick) which is different than the matrix (lamellar) protein. A preparation of the crystal envelopes has been achieved by a papain digestion of the matrix, followed by a dilute acid extraction of the mineral. Studies are in progress on the chemical and physical properties of the envelopes. BIBLIOGRAPHIC REFERENCES: M. Marsh, G. Hopkins, F. Fisher, and R. Sass. "Structure of the Molluscan Bivalve Hinge Ligament, a Unique Calcified Elastic Tissue." (1976) J. Ultrastruct., in press. G. Kahler, R. Sass, and F. Fisher. "The Fine Structure and Crystallography of the Hinge Ligament of Spisula solidissima." (1976) J. Comp. Physiol., in press.